The present invention relates to the manufacture of semiconductor devices, and more particularly, to the manufacture of semiconductor devices requiring multiple oxides with different thickness with high ratios between thick and thin oxide layers such as used in dual gate oxide CMOS devices.
As will be understood by those skilled in the art, processes presently used for manufacturing semiconductor devices having a large ratio between areas of thin oxide and areas of thick oxide with large thickness differences such as used for dual gate devices, typically require an N2 implant and/or a critical resist process.
In N2 implant processes, the wafer is covered with a sacrificial layer, e.g. a thin oxide. Some areas of the wafer are covered with resist, while others are exposed. N2 implantation is performed through a sacrificial oxide in the exposed areas. After resist strip and sacrificial oxide strip, the gate oxidation is performed. The oxide thickness is reduced in N2 implanted areas. The problem with this scheme is that relatively high N2 implant doses are required for large oxide thickness differences, that is, a large ratio between the thick and thin gate oxide. In addition, the effect of oxidation retardation by N2 implant is only efficient at relatively high oxidation temperatures. High temperatures, however, lead to diffusion of the implanted species that are critical for the device definition, resulting in a device degradation. Even with high oxidation temperatures it is difficult to get differences in oxide thickness between the thick oxide and the thin oxide of a factor of 2 or higher.
Alternatively, resist processes are used which allow for arbitrarily large thickness ratios between thin and thick oxide. In a resist process, a first oxide is grown on all silicon surfaces. In the following step, resist is applied. The resist covers some areas of the wafer, while leaving other areas exposed. The oxide is etched in the exposed areas, e.g. by wet chemical oxide etch. After resist strip, a second oxidation is performed. Problems with this scheme are contamination of the oxide layer with residues from the resist or from the resist processing steps.
Therefore, it is an object of this invention to provide a method of forming areas of oxide on a substrate having a thin oxide layer, and other areas of oxide on the substrate with relatively thick oxide layers.
It is another object of the invention to form such thick and thin oxide areas without implanting N2.
It is still another object of the invention to form such thick and thin oxide areas without the use of a critical resist process such that at no time the resist is present on exposed silicon areas before oxidation, or on the final oxide layer.
Other objects and advantages of the invention will in part be obvious, and will in part appear hereinafter, and will be accomplished by the present invention which provides a process for forming a semiconductor circuit on a substrate having first and second areas with different thicknesses of oxide. The process comprises the steps of providing a substrate having at least one or more exposed areas and one or more adjacent areas that are covered by a nitride layer. An oxide having a first thickness is then formed over the exposed areas of the substrate. The nitride layer is then removed from the adjacent areas such that the substrate now has one or more areas with oxide of the first thickness, and one or more newly exposed substrate areas. The newly exposed areas, of course, result from the removal of the nitride layer. An oxide is then formed on the newly exposed areas and has a second thickness. The second thickness is less than the first thickness formed over the first original exposed areas.
According to a first embodiment of the invention, the substrate having the exposed areas or area, and the adjacent area or areas covered by a nitride layer may be formed, according to the first embodiment, by depositing a nitride layer over a surface of the substrate. The layer of nitride is then itself covered by depositing a sacrificial oxide layer over the nitride layer. The sacrificial oxide layer is then patterned by depositing a mask over the sacrificial oxide layer to provide at least one protected area of the oxide layer protected by the mask and to leave at least one area of the sacrificial oxide layer exposed. The exposed area or areas of the sacrificial oxide layer is then removed from the substrate such as by etching. Removal or etching of the sacrificial oxide layer exposes a first area of the nitride layer that was previously covered by the oxide layer.
Subsequent to the etching of the exposed sacrificial oxide layer, the mask is then removed so as to expose the areas of the oxide layer protected by the mask pattern. Thus, at this point, the structure includes patterned areas of the sacrificial oxide layer and adjacent areas covered by a nitride layer. The exposed first areas of the nitride layer are then etched all the way down to the substrate by a process selective to the sacrificial oxide layer to leave exposed areas of the substrate. Of course, since the etching process is selective to the sacrificial oxide, the areas covered by the oxide layer are not etched.
The exposed areas of the oxide layer, which were originally protected by the mask, are removed by a suitable process, thereby leaving the substrate such that it now comprises at least one exposed area of the substrate and at least one adjacent area of the substrate covered by a nitride layer.
As will be discussed hereinafter, a substrate having one or more exposed areas and one or more adjacent areas covered by a nitride layer form a basic structure for providing first areas having an oxide of a first thickness and second areas having a second thickness which is different from the first thickness.
Further, a substrate having one or more exposed areas and one or more adjacent nitride covered areas may also be produced by a second embodiment of the invention. This second embodiment of the invention comprises the steps of depositing a sacrificial oxide layer over the top surface of the substrate and then depositing a first nitride layer over the sacrificial oxide layer. A resist mask is then deposited over the first nitride layer so as to provide at least one protected or patterned area of the first nitride layer, while at the same time, leaving exposed adjacent areas of the first nitride layer. The areas on the substrate having an exposed nitride layer are then subjected to a process for removing the exposed nitride layer so as to expose a first area or areas of the sacrificial oxide layer that were beneath or below the nitride layer. The mask is then stripped to uncover the protected area of the first nitride layer, the first area or areas of the sacrificial oxide layer exposed by the previous etching of the exposed nitride layer are then etched to expose an area or areas of the substrate. The etching of the sacrificial oxide layer is selective to the nitride layer. Therefore, the areas of the substrate that are exposed are those areas that were beneath the sacrificial oxide layer.
The areas of the first nitride layer that were previously protected by the mask are now removed or etched so as to expose second areas of the sacrificial oxide layer which were below the nitride layer. A second nitride layer is then deposited over the exposed area of the substrate so as to form the adjacent areas of the substrate as discussed above. The second areas of the sacrificial oxide layer that were previously covered by the first nitride layer are then removed or etched to form the exposed areas of the substrate.
Therefore, it is seen either the first or second embodiments of the invention may be used to obtain a substrate having one or more exposed areas and one or more adjacent areas covered by a nitride layer. A substrate patterned with first areas with exposed substrate and adjacent areas having a nitride layer is then subjected to a process which forms an oxide having a first thickness over the exposed areas. The adjacent areas having a nitride layer are then stripped to remove the nitride layer and leave adjacent areas of exposed substrate.
Finally, oxide is formed on the adjacent areas of exposed substrate. The oxide forms on the adjacent areas has a second thickness that is less than the thickness of the first oxide.